Resist plays a major role in lithography processes for fabrication of semiconductor devices in which the sizes, as well as positions of transistors, resistors, and interconnects are precisely determined on the wafer and fabricated. By using patterned resist, selective etching and impurity doping can be performed. Resists are not a part of the device structure itself, but rather a masking material to enhance selectivity. It is essential to utilize a resist removal process which removes the resist after it has been utilized but which does not damage the fabricated semiconductor device.
As semiconductor devices become more integrated and miniaturized, ion implantation is extensively employed to accurately control impurity distributions in the semiconductor substrate. The concentration and depth of the dopant impurity is controlled by varying the dose of the dopant, the acceleration energy, and the ion current. During this process, a patterned resist is often used for the selected implant of ions.
Generally, an ion implantation process for fabricating a semiconductor device utilizes a resist layer formed from a resist material typically including a matrix material or resin and a sensitizer which is a photoactive compound (PAC). Photoresists include both positive and negative photoresists. One common photoresist includes diazonaphtoquinone (DQ), which acts as the PAC, and novolak, a phenolic-formaldehyde resin. A mask is utilized to expose portions of the photoresist layer to UV light and then the photoresist layer is developed resulting in a patterned resist layer. The patterned resist layer is used so that only selected portions of an underlying substrate are implanted with ions during ion implantation. The resist layer must be removed after the ion implantation is over before proceeding to the next step in the fabrication of the semiconductor device. Various processes have been used in the past for the removal of the resist. Such processes include, but are not limited to, wet chemical etching processes, e.g., in a mixed solution of sulphuric acid and hydrogen peroxide, and dry plasma etching processes, e.g., in an oxygen plasma ashing process.
However, when high doses of ions, for example, such as doses more than 1.times.10.sup.14 atoms/cm.sup.2, are implanted, the ions are also implanted into the resist layer such that an upper region of the resist layer becomes physically and chemically rigid. The formation of an implanted upper region of the resist layer has been called carbonization and the designated region has been referred to as the carbonized region. However, as indicated in the article "Ion-Implanted Photoresist and Damage-Free Stripping," by Kouichi Hirose, et al., J. Electrochem. Soc., Vol. 141, No. 1, January 1994, the ion-implanted upper region of the resist layer is difficult to remove because the surface has an extremely low hydrogen concentration forming an inactive high-polymer layer as opposed to the carbonization of the surface.
Because of the ion implanted upper region of the resist layer, resist removal, for example, by O.sub.2 plasma ashing, causes damage to the substrate on which the resist is formed. During such ashing, cracks may be generated in the resist film which can lead to sudden and intense ashing of the resist inside and further result in damage to the surface region of the substrate. For example, a gate oxide layer of a semiconductor device fabricated under the resist layer may be damaged or degraded.
As such, additional, complicated, time consuming and costly processes have been required to perform the difficult removal of the resist layer when high doses of ions are implanted because of the resulting low hydrogen concentration of the upper region. One such process is described in U.S. Pat. No. 4,861,424 to Fujimura et al. which utilizes two etching processes to carry out the removal of the resist layer. Further, as described above, if only conventional processes are utilized for the removal of the resist, a fabricated portion of the substrate under the resist layer may be damaged or degraded. Therefore, there is a need in the art for improved ion-implanted resist removal methods.